The bear had been confiscated in Puerto Rico by U.S. Fish and Wildlife Service agents in March 2002 after the circus abandoned her there. When she arrived at the zoo soon after, animal keepers found that she was deaf, overweight and had poor muscle tone, but they nursed her to health and developed a training program using hand signals and other visual cues.

"Alaska was such a remarkable polar bear with a story that touched thousands of people over the course of her life here at the zoo," Don Hutchinson, the zoo's CEO, said in a statement. "Words cannot describe how our entire staff is feeling, in particular the keepers who worked with her every day for years."

Zoologists estimate the bear was in her late 20s. Median life expectancy for female polar bears is about 24 years, zoo officials said, and kidney disease is a common ailment in older bears.

"We had been monitoring her for any signs of lethargy or discomfort, adjusting medications as necessary," said Ellen Bronson, the zoo's chief veterinarian. "Unfortunately, it became apparent that her quality of life was in decline and we knew that the time had come to make the tough decision to euthanize her."

Zoo officials plan to perform a necropsy for scientific research.

Two polar bears remain on exhibit at the zoo; a male named Magnet and a female, Anoki.